Mon, Jul 02, 18

A Guide on Choosing the Best Telescope Eyepieces

A Guide on Choosing the Best Telescope Eyepieces

The optical elements of eyepieces allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. It may seem like a small link in the chain, but it has a large effect on your telescope's optical system, and finding suitable eyepieces will greatly enhance its potential. 

With so many options to choose from, selecting the right set of eyepieces for you and your telescope can seem a little tricky. This guide offers some insight and explanations on different eyepiece types, specifications, and how it all ties together to optimize your astronomy and astrophotography sessions!

Focal Length and Magnification

The Focal Length is an important specification to consider when determining the magnification, also known as power, of an eyepiece and the telescope it's being used with. The following formula will help you determine the magnification based on your eyepiece and telescope's specifications:

Magnification  = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)
 
For example:
  • A 20 mm eyepiece on a 2000 mm telescope (2000/20) gives you 100 power (100x), this makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye. 
Note: When using your telescope at different powers, you generally have a choice of a small, sharp, and bright image at lower magnification; or a larger, yet blurred and dim image at higher magnification. The reason being, that the telescope gathers a fixed amount of light, and at higher magnifications, the same amount of light is being spread over a larger area, resulting in a dimmer image.

    Field of View: Apparent and True

    An eyepiece's Apparent Field of View (AFOV) is expressed in degrees (°). It is how much of the sky is seen edge-to-edge through the eyepiece alone. AFOV's range from narrow (25° - 30°) to an extra-wide angle (80° or more).

    An eyepiece's true field of view is the angle of sky seen through the eyepiece when it's attached to the telescope. The true field can be calculated using the following formula:

    True Field = Apparent Field / Magnification

    For example, suppose you have an 8-inch Schmidt-Cassegrain telescope with a 2000 mm focal length and a 20 mm eyepiece with a 50° apparent field. The magnification would be 2000 mm / 20 mm = 100x. The true field would be 50\100, or 0.5° - about the same apparent diameter as the full moon.



     
    Eye Relief and Corrective Lenses

    Eye Relief refers to the distance between your eye and the eyepiece lens when the image is in focus. Eye relief is traditionally in proportion with focal length: The shorter the focal length, the shorter the eye relief. However, some of the more modern eyepiece designs provide long-eye relief regardless of focal length, which is especially beneficial to those who wear glasses. If you like to keep your glasses on while using a telescope, the eye relief of an eyepiece is an important specification to consider (we recommend looking at long-eye relief eyepieces). 

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    2 mm - 4.9 mm Eyepieces

    These provide very high magnifications and work best on long focal length refractors and Schmidt-Cassegrains. Unless you have very steady seeing conditions, this range more than likely will produce too much magnification for other telescope types.

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    5 mm - 6.9 mm Eyepieces

    These make good planetary detail and double star eyepieces for long focal length telescopes and will work satisfactorily in shorter focal length telescopes with steady seeing conditions.

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    7 mm - 9.9 mm Eyepieces

    Ideal high magnification eyepieces for shorter focal length telescopes and serve as good planetary, double star, and lunar detail units.

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    10 mm - 13.9 mm Eyepieces

    Good to use across all focal lengths and offer great background darkening capabilities for studying planetary nebula, small galaxies, planetary details, and lunar details.

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    14 mm - 17.9 mm Eyepieces

    A great mid-range magnification for all focal lengths and helps resolve globular clusters, galaxy details, and spot planetary nebulae.

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    18 mm - 24.9 mm Eyepieces

    Works nicely on long focal length telescopes to show wide field and extended objects. Shorter focal length telescopes will enjoy great mid-range magnification of galaxy clusters and large open clusters.

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    25 mm - 30.9  mm Eyepieces

    Longer focal lengths are good for large nebula and open clusters. Shorter focal lengths are great for large objects such as the Orion nebula, views of the full lunar disc, large open clusters, and more. It also makes for good "locator" eyepieces in all focal lengths.

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    31 mm - 39.9 mm Eyepieces

    These are well suited for shorter focal length telescopes for extended views and large, starry fields.

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    40 mm Eyepieces

    These are exclusively the domain of shorter focal length telescopes. This magnification range is superb for showing large, starry vistas as well as extended nebulae with star fields, etc.

    How Exit Pupil Relates to Power

    Exit pupil refers to the size of the bundle of light rays coming out of the eyepiece. Exit pupil size (in inches) can be calculated by:

    Exit pupil size (mm) = Telescope aperture (mm) / Telescope magnification
    or
    Exit pupil size (mm) = Eyepiece focal length (mm) / Telescope f-ratio

     

    In order for all the light rays to enter your pupil, the exit pupil must be smaller than the pupil of your eye. A young person's fully dark-adapted eyes may have 7 mm-wide pupils. As you age, the maximum pupil diameter decreases. For middle-aged adults, the practical maximum is closer to 5 mm.
    At the other end of the scale, magnifications that yield an exit pupil in the range of 0.5 mm to 1.0 mm, empty magnification begins to set in, depending on the quality of your telescope and your eyes. In other words, this much magnification starts to degrade the image you see.

    How Many Eyepieces Do I Really Need

    Although there is no specific number of eyepieces you should own, with a few different telescope eyepieces, you have a better chance of hitting the optimal power for the particular object you are observing, given the sky conditions at the time. Usually, you'll want to start with low power (i.e., long eyepiece focal length, such as 25 mm or 30 mm) to get the object in the field of view of the telescope. Then you might want to try a slightly higher-power (shorter focal length, maybe 18 mm or 15 mm) eyepiece and see if the view looks any better. If it does, swap in an even higher-power eyepiece, etc., until you hit that "sweet spot" where image brightness, image scale, and the amount of visible detail combine to form the most pleasing view. 

    What about Barlow Lenses?

    You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers to increase power, such as a Barlow lens. A Barlow increases the effective focal length of an objective lens, increasing the magnification. The idea is that two eyepieces and a Barlow will give you the flexibility of magnification of four eyepieces, and will give higher magnifications with less powerful eyepieces.

    Using different eyepieces can profoundly increase the versatility and functionality of any telescope. 

    Basic Tips to Follow When Shopping for Eyepieces

    • Consider the focal length of your telescope, or telescopes, to make sure the eyepiece will provide an appropriate magnification to suit your needs.

    • If you wear eyeglasses while using a telescope, pay attention to the eye relief specification of different eyepieces, as ample eye relief can improve comfort and ease-of-use while wearing corrective lenses.

    • Depending on your observing goals, consider the apparent field of view of your eyepiece choices.

    • If versatility is paramount, consider a zoom eyepiece or Barlow lens to increase the number of possible magnifications to use.


    21 comments

    • Richard

      I have a Discovery 15" f/5 Truss design with a 75" focal length. I am self taught and have never had a great viewing experience. I am hoping if I buy some new eyepieces my viewing experience will greatly improve. I am not sure which ones to buy. I currently own 3 eye pieces Meade Super Plossl 56mm, Meade Ultra Wide Angle 8.8 mm, and a TeleVue 35mm Panoptic. I would like to view the planets with better clarity. I am interested in buying a few new eyepieces. Which ones can you recommend for my telescope? Thanks

    • John

      Hi all and thanks for this great article — very helpful. I want to echo the question Lou just asked (a few days ago). I had a telescope as a kid and during COVID decided to renew my interest in astronomy so a Celestron 114LZ (1000mm focal — f/9 aperture) is what I bought. I was really hoping to see the basic stuff — the great red spot, defined rings of Saturn and most of all — Andromeda. I live in Indio in the Coachella Valley and have very good viewing conditions most of the time.
      Well, Jupiter looks like an overexposed white disk (I can see 4 moons), I can barely make out the rings of Saturn (10mm eyepiece) and last night Andromeda was just a fuzzy blur (10mm eyepiece).
      To Lou’s question — should I be looking at a new 10mm eyepiece of higher quality and/or a new Barlow with higher than 2x magnification? Any suggestions as to what brands of eyepieces are best?
      Many thanks!

      John

    • Lou

      Great article! My question is are more expensive eyepieces necessarily better or give a sharper image? I use the inexpensive Plossl eyepieces but have no real complaints with them. However, I hear from other astronomers in blogs, etc. that the eyepieces they use, which are usually more expensive, give sharper views. Sometimes I feel I may be missing out on sharper views by using the inexpensive ones. Thx!

    • WillIam Saldana

      I just purchased Gsyker 70400 telescope and I am looking for better definition I really want to use this telescope and learn. I believe if I learn on this telescope I will buy a better one.

    • OPT

      Hey Gene, congratulations on your purchase on the Celestron 11 SCT AVX! It’s a powerful scope. A Barlow lens is a nice upgrade to eyepiece collections. For other eyepieces, any eyepieces between 18 mm and 31.9 mm will work. Clear skies!

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